1. Field of the Invention
The present invention relates generally to a rotating magnetic disk drive (disk drive), and more particularly to a method of seamlessly recording circumferentially successive servo bursts that overlap one another for providing servo information with an enhanced signal-to-noise ratio (SNR).
2. Description of the Related Art
A conventional disk drive has a head disk assembly ("HDA") including at least one magnetic disk ("disk"), a spindle motor for rapidly rotating the disk, and a head stack assembly ("HSA") that includes a transducer head for reading and writing data. The transducer head is controllably positioned by a servo system in order to read or write information from or to particular tracks on the disk. The typical HSA has two primary portions: (1) an actuator assembly that moves in response to the servo control system and (2) a head gimbal assembly ("HGA") that extends from the actuator assembly and biases the head toward the disk. The typical HSA further includes a flex circuit on the side of the actuator body and electrical conductors which run between the flex circuit and the head to transmit read and write signals to and from the head.
The industry presently prefers a "rotary" or "swing-type" actuator assembly which conventionally comprises an actuator body that rotates on a pivot assembly between limited positions, a coil that extends from one side of the actuator body to interact with permanent magnets to form a voice coil motor, and an actuator arm that extends from the opposite side of the actuator body to support the HGA.
Each surface of each disk conventionally contains a plurality of concentric tracks, each track comprising a plurality of servo data segments and user data segments. The servo data and user data of adjacent concentric tracks are radially aligned to form a plurality of sectors containing servo wedges and corresponding data wedges. The servo data comprises track ID fields and servo bursts (an alternating pattern of magnetic transitions) which are used by the servo system to align the transducer head with a particular data track. The servo control system moves the transducer head toward a desired track during a coarse position or "seek" mode based on the track ID field. Once the transducer head is over the desired track, the servo control system enters a fine position or "track follow" mode and uses the servo bursts to keep the transducer head over the data.
For many years, the industry has used inductive heads where the same transducer is used for reading and writing. More recently, however, the industry has begun using magnetoresistive transducers which are only capable of reading. Therefore, two separate heads are required--an inductive head for writing and a magnetoresistive head for reading. The separate read and write heads are necessarily spaced from one another--usually one behind the other. A variable skew between the two heads is introduced by this spacing due to the tangential relationship of the transducers to a circular data track on the disk as they are positioned over the disk from inner to outer tracks. The relative positions of the two heads may be set during the manufacturing process to bias the skew so that the heads may be, for example, aligned when positioned over the innermost tracks and skewed when positioned over the outermost tracks or other arrangements suitable for a particular design objective. Manufacturing tolerances can also cause a small shift in the relative centerlines of the heads with respect to one another.
A problem exists with using the magnetoresistive transducer head, therefore, because the servo bursts are read with the magnetoresistive read head but the data tracks are written with the inductive write head that is variably skewed from the read head depending on the radial position over the disk. As a result of this physical displacement between the two heads, it is necessary to offset or "microjog" the transducer head during the read operation or during the write operation.
The servo electronics convert the amplitude of each burst to an electrical signal to generate a position error signal (PES) that a microprocessor uses to determine the required control effort or correction needed to track follow. In this disclosure, the equal signal center line of a burst pair is called a "burst pair centerline." The servo control system usually aligns the read head with a burst pair centerline while writing, so that the centerline of the data track is displaced from the burst pair centerline by the physical displacement between the read and write heads at that particular radius. Later, in order to maximize the signal amplitude and the signal-to-noise ratio when reading the data, the servo control system micro-jogs the read head away from the burst pair centerline in the same direction as the original physical displacement so that the read head passes over the center line of the data track while track following.
A continuing problem is that the magnetoresistive read head is relatively narrow and has a limited range of linearity. In other words, as the read head is displaced from the burst pair centerline, there is a relatively small amount of displacement over which the signal produced by the servo bursts remains linearly related. In the inventors'experience, the magnetoresistive read head can be conventionally displaced from the burst pair centerline by about 1/6 of a track pitch (the distance between centerlines of adjacent tracks) and still remain within a useable range of linearity.
One potential approach to resolving the narrow range of linearity of the magnetoresistive read head is providing additional burst pair centerlines within a data track pitch so that the read head is always within its linear width of at least one of the burst pair centerlines. In a conventional four burst servo pattern comprising full data track width (100%) servo bursts forming A/B burst pairs and C/D burst pairs that are arranged in "quadrature" to one another, there are two burst pair centerlines per data track pitch. The burst pair centerlines occur, therefore, at one-half track intervals. Since the magnetoresistive read transducer cannot be micro-jogged more than 25% of a data track pitch and remain within its linear range, it may be desirable to provide more than two burst pair centerlines per data track pitch by adding more servo bursts in a circumferential dimension or in a radial dimension. If we wanted three burst pair centerlines per data track pitch, for example, we could add more servo bursts in the circumferential dimension by adding another burst pair, e.g. an E/F burst pair, and then position the burst pair centerlines of the A/B pair, the C/D pair, and the E/F pair at 1/3 data track pitch offsets. However, an additional servo burst pair in the circumferential dimension takes up valuable space that could otherwise be occupied by data. It is desirable, therefore, to increase the number of servo bursts in the radial direction rather than the circumferential direction without restricting the number of data tracks on a surface. We can accomplish this by using only two burst pairs where each burst is sufficiently narrow so that three burst pair centerlines occur within each data track pitch.
A device called a servo track writer or "servowriter" is generally used to record the servo bursts and other servo information. The servowriter is basically a jig that mechanically moves the heads to a desired radial position and then causes a portion of the servo information to be recorded for that position. The servowriting process conventionally records a plurality of servo sectors by writing track identification data and servo bursts at discrete intervals around the track circumference. The servowriter then steps the headstack position by a fraction of a track pitch, and then may write new bursts and overwrite or erase a portion of previously written bursts. Overwriting a portion of a burst serves to extend its width, while erasing a portion of a burst "trims" the burst to a desired width. Each stepping of the headstack and subsequent writing of bursts or erasures is commonly termed a "pass" of the servowriter. Since the stepping is significantly less than the burst width, the bursts written during a current pass may be said to "overlap" bursts from a previous pass.
In a conventional servo writing process, the write current is always turned ON when a servo sector is passing under the head such that the write head is either writing (recording magnetic transitions) or erasing (the current being held in a steady state direction). When a given servo burst is overwritten after stepping, this always ON condition of the write current inherently causes the servo burst to be "stitched" together in two write passes and then it must conventionally be trimmed in a third erase pass. Servo bursts recorded with two write passes have a central artifact that is caused by erase bands created at the outer edges of the write head. These bands form a "seam" where the burst is stitched together. This seam, sometimes called "hooks" or "transition hooks" contributes noise during the reading of servo bursts. This noise, occurring during the reading of servo burst signals, is deleterious to an important measure of quality in servo processing--the Signal-to-Noise Ratio or SNR. The effects of the noise are more pronounced in the case of MR heads because of the non-linear response characteristic of the heads. Also, hooks are a relatively minor problem when servo burst widths are 100% of a data track because they occupy a relatively small percentage of the burst portion being read (conventionally 50% of a track pitch) and, moreover, because the hooks are so far from the burst pair centerline that a major portion of the read head would generally not pass over the hooks in a track follow mode. With the narrower bursts often used with MR heads, the hooks are more significant.
Another artifact of the stitched servo burst is a slight phase offset between the stitched portions of the burst. This phase offset, termed "shingling", can contribute errors in processing servo burst amplitudes where the transitions within a burst are integrated to define a burst amplitude.
Given the above disadvantages of creating servo burst patterns which are stitched together to form seams and the need to provide servo burst patterns which are compatible with MR heads there is a clear need to provide to provide a unique method of seamlessly recording servo bursts that circumferentially overlap one another without creating hooks or shingling effects.